As modern electronic circuit boards evolve toward increased circuit and component densities, thorough board cleaning after soldering becomes a more important criterion.
Current industrial processes for soldering electronic components to circuit boards involve coating the entire circuit side of the board with flux and thereafter passing the flux-coated board over preheaters and through molten solder. The flux cleans the conductive metal parts and promotes solder fusion. Commonly used solder fluxes generally consist of rosin, either used alone or with activating additives, such as amine hydrochlorides or oxalic acid derivatives.
After soldering, which thermally degrades part of the rosin, the flux-residues are often removed from the circuit boards with an organic solvent. The requirements for such solvents are very stringent. Defluxing solvents should have the following characteristics: a low boiling point, be nonflammable, have low toxicity and have high solvency power, so that flux and flux-residues can be removed without damaging the substrate being cleaned. While boiling point, flammability and solvent power characteristics can often be adjusted by preparing solvent mixtures, these mixtures are often unsatisfactory because they fractionate to an undesirable degree during use. Such solvent mixtures also fractionate during solvent distillation, which makes it virtually impossible to recover a solvent mixture with the original composition.
On the other hand, azeotropic mixtures, with their constant boiling points and constant compositions, have been found to be very useful for these applications. Azeotropic mixtures exhibit either a maximum or minimum boiling point and they do not fractionate on boiling. These characteristics are also important when using solvent compositions to remove solder fluxes and flux-residues from printed circuit boards.
Preferential evaporation of the more volatile solvent mixture components would occur, if the mixtures were not azeotropic and would result in mixtures with changed compositions, and with attendant less-desirable solvency properties, such as lower rosin flux solvency and lower inertness toward the electrical components being cleaned. The azeotropic character is also desirable in vapor degreasing operations, where redistilled solvent is generally employed for final rinse cleaning.
In summary, vapor defluxing and degreasing systems act as a still. Unless the solvent composition exhibits a constant boiling point, i.e., is azeotropic fractionation will occur and undesirable solvent distributions will result, which could detrimentally affect the safety and efficacy of the cleaning operation.
A number of chlorofluorocarbon based azeotropic compositions have been discovered and in some cases used as solvents for solder flux and flux-residue removal from printed circuit boards and also for miscellaneous degreasing applications. For example: U.S. Pat. No. 3,903,009 discloses the ternary azeotrope of 1,1,2-trichlorotrifluoroethane with ethanol and nitromethane; U.S. Pat. No. 2,999,815 discloses the binary azeotrope of 1,1,2-trichlorotrifluoroethane and acetone U.S. Pat. No. 2,999,817 discloses the binary azeotrope of 1,1,2-trichlorotrifluoroethane and methylene chloride.
Such mixtures are also useful as buffing abrasive detergents, for the removal of buffing abrasive components from polished surfaces, as drying agents for cleaned polished surfaces such as jewelry and metals and as resist-developers in conventional circuit manufacturing techniques, which employ chlorine-type developing agents. The mixtures are also useful as refrigerants, heat transfer media, foam expansion agents, aerosol propellants, solvents and power cycle working fluids. Further, in many cases, the halocarbon component of the azeotropic mixture itself would be effective in these applications.
Closed-cell polyurethane foams are widely used for insulation purposes in building construction and in the manufacture of energy efficient electrical appliances. Polyisocyanurate board stock is used by the construction industry, in roofing and siding for both its insulative and load-carrying capabilities. Sprayed polyurethane foams are also used in construction for the insulation of large structures such as storage tanks. Pour-in-place polyurethane foams are used as insulative barriers in appliances such as refrigerators and freezers and also in much larger items such as trailer and railroad tanks. All of the aforementioned types of expandable foam require the use of expansion agents (blowing agents) for their manufacture. Insulative foams require the use of halocarbon blowing agents, owing to their low vapor thermal conductivities, not only to expand the polymer but also to impart the essential insulation feature to the expanded foam. Historically, polyurethane and polyisocyanurate foams have been produced using trichlorofluoromethane (CFC-11), as the blowing agent of choice.
Another important type of insulative foam is the phenolic foam. These foams have heretofore generally been expanded with blends of trichlorofluoromethane (CFC-11) and 1,1,2-trichlorotrifluoroethane (CFC-113) blowing agents.
Still another insulating foam is the thermoplastic or polyolefin type foam. These are generally the polyethylene and polypropylene foams, used widely in packaging. Thermoplastic foams are usually expanded with dichlorodifluoromethane (CFC-12).
Many smaller scale hermetically sealed, refrigeration systems, such as those used in refrigerators or window and auto air conditioners, use dichlorodifluoromethane (CFC-12) as the refrigerant.
Larger scale centrifugal refrigeration equipment, such as those used for industrial scale cooling, e.g, commercial office buildings, generally employ trichlorofluoromethane (CFC-11) or 1,1,2-trichlorotrifluoroethane (CFC-113) as the refrigerants of choice. Azeotropic mixtures, with their constant boiling points and compositions have also been found to be very useful as substitute refrigerants, for the applications cited above.
Aerosol products have employed both individual halocarbons and halocarbon blends as propellant systems. Halocarbons have also been used both as solvents and propellant vapor pressure attenuators, in aerosol systems. Azeotropic mixtures, with their constant compositions and vapor pressures would be very useful as solvents and propellants in aerosol systems.
Some of the chlorofluorocarbons which are currently used as cleaning agents, blowing agents, refrigerants, aerosol propellants and for other applications, have been linked, theoretically, to depletion of the earth's protective ozone layer. As early as the mid-1970's, it was known that introduction of hydrogen atoms into the chemical structure of previously fully-halogenated chlorofluorocarbons would reduce the chemical stability of these compounds. Hence, the now destabilized compounds would be expected to degrade in the lower atmosphere and not reach the stratospheric ozone layer in-tact. What is also needed, therefore, are substitute halocarbons, which have low theoretical ozone depletion potentials.
Unfortunately, as recognized in the art, it is not possible to predict the formation of azeotropes. This fact complicates the search for new azeotropic compositions, which have application in the field. Nevertheless, there is a constant effort in the art to discover new azeotropic compositions, which have desirable end-use characteristics.